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Network Working Group S. Weiler
Internet-Draft SPARTA, Inc
Updates: 4034, 4035 (if approved) May 23, 2005
Expires: November 24, 2005
Clarifications and Implementation Notes for DNSSECbis
draft-ietf-dnsext-dnssec-bis-updates-01
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 24, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document is a collection of minor technical clarifications to
the DNSSECbis document set. It is meant to serve as a resource to
implementors as well as an interim repository of possible DNSSECbis
errata.
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Proposed additions in future versions
An index sorted by the section of DNSSECbis being clarified.
A list of proposed protocol changes being made in other documents,
such as NSEC3 and Epsilon. This document would not make those
changes, merely provide an index into the documents that are making
changes.
Changes between -00 and -01
Document significantly restructured.
Added section on QTYPE=ANY.
Changes between personal submission and first WG draft
Added Section 2.1 based on namedroppers discussions from March 9-10,
2005.
Added Section 3.4, Section 3.3, Section 4.3, and Section 2.2.
Added the DNSSECbis RFC numbers.
Figured out the confusion in Section 4.1.
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Table of Contents
1. Introduction and Terminology . . . . . . . . . . . . . . . . . 4
1.1 Structure of this Document . . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Significant Concerns . . . . . . . . . . . . . . . . . . . . . 4
2.1 Clarifications on Non-Existence Proofs . . . . . . . . . . 4
2.2 Empty Non-Terminal Proofs . . . . . . . . . . . . . . . . 5
2.3 Validating Responses to an ANY Query . . . . . . . . . . . 5
3. Interoperability Concerns . . . . . . . . . . . . . . . . . . 5
3.1 Unknown DS Message Digest Algorithms . . . . . . . . . . . 5
3.2 Private Algorithms . . . . . . . . . . . . . . . . . . . . 6
3.3 Caution About Local Policy and Multiple RRSIGs . . . . . . 6
3.4 Key Tag Calculation . . . . . . . . . . . . . . . . . . . 7
4. Minor Corrections and Clarifications . . . . . . . . . . . . . 7
4.1 Finding Zone Cuts . . . . . . . . . . . . . . . . . . . . 7
4.2 Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 7
4.3 Errors in Examples . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Normative References . . . . . . . . . . . . . . . . . . . 8
7.2 Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction and Terminology
This document lists some minor clarifications and corrections to
DNSSECbis, as described in [1], [2], and [3].
It is intended to serve as a resource for implementors and as a
repository of items that need to be addressed when advancing the
DNSSECbis documents from Proposed Standard to Draft Standard.
In this version (-01 of the WG document), feedback is particularly
solicited on the structure of the document and whether the text in
the recently added sections is correct and sufficient.
Proposed substantive additions to this document should be sent to the
namedroppers mailing list as well as to the editor of this document.
The editor would greatly prefer text suitable for direct inclusion in
this document.
1.1 Structure of this Document
The clarifications to DNSSECbis are sorted according to the editor's
impression of their importance, starting with ones which could, if
ignored, lead to security and stability problems and progressing down
to clarifications that are likely to have little operational impact.
Mere typos and awkward phrasings are not addressed unless they could
lead to misinterpretation of the DNSSECbis documents.
1.2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4].
2. Significant Concerns
This section provides clarifications that, if overlooked, could lead
to security issues or major interoperability problems.
2.1 Clarifications on Non-Existence Proofs
RFC4035 Section 5.4 slightly underspecifies the algorithm for
checking non-existence proofs. In particular, the algorithm there
might incorrectly allow the NSEC from the parent side of a zone cut
to prove the non-existence of either other RRs at that name in the
child zone or other names in the child zone. It might also allow a
NSEC at the same name as a DNAME to prove the non-existence of names
beneath that DNAME.
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A parent-side delegation NSEC (one with the NS bit set, but no SOA
bit set, and with a singer field that's shorter than the owner name)
must not be used to assume non-existence of any RRs below that zone
cut (both RRs at that ownername and at ownernames with more leading
labels, no matter their content). Similarly, an NSEC with the DNAME
bit set must not be used to assume the non-existence of any
descendant of that NSEC's owner name.
2.2 Empty Non-Terminal Proofs
To be written, based on Roy Arends' May 11th message to namedroppers.
2.3 Validating Responses to an ANY Query
RFC4035 does not address now to validate responses when QTYPE=*. As
described in Section 6.2.2 of RFC1034, a proper response to QTYPE=*
may include a subset of the RRsets at a given name -- it is not
necessary to include all RRsets at the QNAME in the response.
When validating a response to QTYPE=*, validate all received RRsets
that match QNAME and QCLASS. If any of those RRsets fail validation,
treat the answer as Bogus. If there are no RRsets matching QNAME and
QCLASS, validate that fact using the rules in RFC4035 Section 5.4 (as
clarified in this document). To be clear, a validator must not
insist on receiving all records at the QNAME in response to QTYPE=*.
3. Interoperability Concerns
3.1 Unknown DS Message Digest Algorithms
Section 5.2 of RFC4035 includes rules for how to handle delegations
to zones that are signed with entirely unsupported algorithms, as
indicated by the algorithms shown in those zone's DS RRsets. It does
not explicitly address how to handle DS records that use unsupported
message digest algorithms. In brief, DS records using unknown or
unsupported message digest algorithms MUST be treated the same way as
DS records referring to DNSKEY RRs of unknown or unsupported
algorithms.
The existing text says:
If the validator does not support any of the algorithms listed
in an authenticated DS RRset, then the resolver has no supported
authentication path leading from the parent to the child. The
resolver should treat this case as it would the case of an
authenticated NSEC RRset proving that no DS RRset exists, as
described above.
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To paraphrase the above, when determining the security status of a
zone, a validator discards (for this purpose only) any DS records
listing unknown or unsupported algorithms. If none are left, the
zone is treated as if it were unsigned.
Modified to consider DS message digest algorithms, a validator also
discards any DS records using unknown or unsupported message digest
algorithms.
3.2 Private Algorithms
As discussed above, section 5.2 of RFC4035 requires that validators
make decisions about the security status of zones based on the public
key algorithms shown in the DS records for those zones. In the case
of private algorithms, as described in RFC4034 Appendix A.1.1, the
eight-bit algorithm field in the DS RR is not conclusive about what
algorithm(s) is actually in use.
If no private algorithms appear in the DS set or if any supported
algorithm appears in the DS set, no special processing will be
needed. In the remaining cases, the security status of the zone
depends on whether or not the resolver supports any of the private
algorithms in use (provided that these DS records use supported hash
functions, as discussed in Section 3.1). In these cases, the
resolver MUST retrieve the corresponding DNSKEY for each private
algorithm DS record and examine the public key field to determine the
algorithm in use. The security-aware resolver MUST ensure that the
hash of the DNSKEY RR's owner name and RDATA matches the digest in
the DS RR. If they do not match, and no other DS establishes that
the zone is secure, the referral should be considered BAD data, as
discussed in RFC4035.
This clarification facilitates the broader use of private algorithms,
as suggested by [5].
3.3 Caution About Local Policy and Multiple RRSIGs
When multiple RRSIGs cover a given RRset, RFC4035 Section 5.3.3
suggests that "the local resolver security policy determines whether
the resolver also has to test these RRSIG RRs and how to resolve
conflicts if these RRSIG RRs lead to differing results." In most
cases, a resolver would be well advised to accept any valid RRSIG as
sufficient. If the first RRSIG tested fails validation, a resolver
would be well advised to try others, giving a successful validation
result if any can be validated and giving a failure only if all
RRSIGs fail validation.
If a resolver adopts a more restrictive policy, there's a danger that
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properly-signed data might unnecessarily fail validation, perhaps
because of cache timing issues. Furthermore, certain zone management
techniques, like the Double Signature Zone-signing Key Rollover
method described in section 4.2.1.2 of [6] might not work reliably.
3.4 Key Tag Calculation
RFC4034 Appendix B.1 incorrectly defines the Key Tag field
calculation for algorithm 1. It correctly says that the Key Tag is
the most significant 16 of the least significant 24 bits of the
public key modulus. However, RFC4034 then goes on to incorrectly say
that this is 4th to last and 3rd to last octets of the public key
modulus. It is, in fact, the 3rd to last and 2nd to last octets.
4. Minor Corrections and Clarifications
4.1 Finding Zone Cuts
Appendix C.8 of RFC4035 discusses sending DS queries to the servers
for a parent zone. To do that, a resolver may first need to apply
special rules to discover what those servers are.
As explained in Section 3.1.4.1 of RFC4035, security-aware name
servers need to apply special processing rules to handle the DS RR,
and in some situations the resolver may also need to apply special
rules to locate the name servers for the parent zone if the resolver
does not already have the parent's NS RRset. Section 4.2 of RFC4035
specifies a mechanism for doing that.
4.2 Clarifications on DNSKEY Usage
Questions of the form "can I use a different DNSKEY for signing the
X" have occasionally arisen.
The short answer is "yes, absolutely". You can even use a different
DNSKEY for each RRset in a zone, subject only to practical limits on
the size of the DNSKEY RRset. However, be aware that there is no way
to tell resolvers what a particularly DNSKEY is supposed to be used
for -- any DNSKEY in the zone's signed DNSKEY RRset may be used to
authenticate any RRset in the zone. For example, if a weaker or less
trusted DNSKEY is being used to authenticate NSEC RRsets or all
dynamically updated records, that same DNSKEY can also be used to
sign any other RRsets from the zone.
Furthermore, note that the SEP bit setting has no effect on how a
DNSKEY may be used -- the validation process is specifically
prohibited from using that bit by RFC4034 section 2.1.2. It possible
to use a DNSKEY without the SEP bit set as the sole secure entry
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point to the zone, yet use a DNSKEY with the SEP bit set to sign all
RRsets in the zone (other than the DNSKEY RRset). It's also possible
to use a single DNSKEY, with or without the SEP bit set, to sign the
entire zone, including the DNSKEY RRset itself.
4.3 Errors in Examples
The text in RFC4035 Section C.1 refers to the examples in B.1 as
"x.w.example.com" while B.1 uses "x.w.example". This is painfully
obvious in the second paragraph where it states that the RRSIG labels
field value of 3 indicates that the answer was not the result of
wildcard expansion. This is true for "x.w.example" but not for
"x.w.example.com", which of course has a label count of 4
(antithetically, a label count of 3 would imply the answer was the
result of a wildcard expansion).
The first paragraph of RFC4035 Section C.6 also has a minor error:
the reference to "a.z.w.w.example" should instead be "a.z.w.example",
as in the previous line.
5. IANA Considerations
This document specifies no IANA Actions.
6. Security Considerations
This document does not make fundamental changes to the DNSSEC
protocol, as it was generally understood when DNSSECbis was
published. It does, however, address some ambiguities and omissions
in those documents that, if not recognized and addressed in
implementations, could lead to security failures. In particular, the
validation algorithm clarifications in Section 2 are critical for
preserving the security properties DNSSEC offers. Furthermore,
failure to address some of the interoperability concerns in Section 3
could limit the ability to later change or expand DNSSEC, including
by adding new algorithms.
7. References
7.1 Normative References
[1] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
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[3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
7.2 Informative References
[5] Blacka, D., "DNSSEC Experiments",
draft-blacka-dnssec-experiments-00 (work in progress),
December 2004.
[6] Gieben, R. and O. Kolkman, "DNSSEC Operational Practices",
draft-ietf-dnsop-dnssec-operational-practices-04 (work in
progress), May 2005.
Author's Address
Samuel Weiler
SPARTA, Inc
7075 Samuel Morse Drive
Columbia, Maryland 21046
US
Email: weiler@tislabs.com
Appendix A. Acknowledgments
The editor is extremely grateful to those who, in addition to finding
errors and omissions in the DNSSECbis document set, have provided
text suitable for inclusion in this document.
The lack of specificity about handling private algorithms, as
described in Section 3.2, and the lack of specificity in handling ANY
queries, as described in Section 2.3, were discovered by David
Blacka.
The error in algorithm 1 key tag calculation, as described in
Section 3.4, was found by Abhijit Hayatnagarkar. Donald Eastlake
contributed text for Section 3.4.
The bug relating to delegation NSEC RR's in Section 2.1 was found by
Roy Badami. Roy Arends found the related problem with DNAME.
The errors in the RFC4035 examples were found by Roy Arends, who also
contributed text for Section 4.3 of this document.
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The editor would like to thank Olafur Gudmundsson and Scott Rose for
their substantive comments on the text of this document.
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Intellectual Property Statement
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Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
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specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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Network Working Group S. Weiler
Internet-Draft SPARTA, Inc.
Updates: 4033, 4034, 4035, 5155 D. Blacka
(if approved) VeriSign, Inc.
Intended status: Standards Track January 14, 2009
Expires: July 18, 2009
Clarifications and Implementation Notes for DNSSECbis
draft-ietf-dnsext-dnssec-bis-updates-08
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 18, 2009.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Weiler & Blacka Expires July 18, 2009 [Page 1]
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Abstract
This document is a collection of technical clarifications to the
DNSSECbis document set. It is meant to serve as a resource to
implementors as well as a repository of DNSSECbis errata.
Table of Contents
1. Introduction and Terminology . . . . . . . . . . . . . . . . . 3
1.1. Structure of this Document . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Important Additions to DNSSSECbis . . . . . . . . . . . . . . 3
2.1. NSEC3 Support . . . . . . . . . . . . . . . . . . . . . . 3
2.2. SHA-256 Support . . . . . . . . . . . . . . . . . . . . . 3
3. Significant Concerns . . . . . . . . . . . . . . . . . . . . . 4
3.1. Clarifications on Non-Existence Proofs . . . . . . . . . . 4
3.2. Validating Responses to an ANY Query . . . . . . . . . . . 4
3.3. Check for CNAME . . . . . . . . . . . . . . . . . . . . . 5
3.4. Insecure Delegation Proofs . . . . . . . . . . . . . . . . 5
3.5. Errors in Canonical Form Type Code List . . . . . . . . . 5
4. Interoperability Concerns . . . . . . . . . . . . . . . . . . 5
4.1. Unknown DS Message Digest Algorithms . . . . . . . . . . . 5
4.2. Private Algorithms . . . . . . . . . . . . . . . . . . . . 6
4.3. Caution About Local Policy and Multiple RRSIGs . . . . . . 6
4.4. Key Tag Calculation . . . . . . . . . . . . . . . . . . . 7
4.5. Setting the DO Bit on Replies . . . . . . . . . . . . . . 7
4.6. Setting the AD bit on Replies . . . . . . . . . . . . . . 7
4.7. Setting the CD bit on Requests . . . . . . . . . . . . . . 8
4.8. Nested Trust Anchors . . . . . . . . . . . . . . . . . . . 8
5. Minor Corrections and Clarifications . . . . . . . . . . . . . 8
5.1. Finding Zone Cuts . . . . . . . . . . . . . . . . . . . . 8
5.2. Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 8
5.3. Errors in Examples . . . . . . . . . . . . . . . . . . . . 9
5.4. Errors in RFC 5155 . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction and Terminology
This document lists some clarifications and corrections to DNSSECbis,
as described in [RFC4033], [RFC4034], and [RFC4035].
It is intended to serve as a resource for implementors and as a
repository of items that need to be addressed when advancing the
DNSSECbis documents from Proposed Standard to Draft Standard.
1.1. Structure of this Document
The clarifications to DNSSECbis are sorted according to their
importance, starting with ones which could, if ignored, lead to
security and stability problems and progressing down to
clarifications that are expected to have little operational impact.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Important Additions to DNSSSECbis
This section provides
2.1. NSEC3 Support
[RFC5155] describes the use and behavior of the NSEC3 and NSEC3PARAM
records for hashed denial of existence. Validator implementations
are strongly encouraged to include support for NSEC3 as a number of
highly visible zones are expected to use it. Validators that do not
support validation of responses using NSEC3 will likely be hampered
in validating large portions of the DNS space.
[RFC5155] should be considered part of the DNS Security Document
Family as described by [RFC4033], Section 10.
2.2. SHA-256 Support
[RFC4509] describes the use of SHA-256 as a digest algorithm for use
with Delegation Signer (DS) RRs. [I-D.ietf-dnsext-dnssec-rsasha256]
describes the use of the RSASHA256 algorthim for use in DNSKEY and
RRSIG RRs. Validator implementations are strongly encouraged to
include support for this algorithm for DS, DNSKEY, and RRSIG records.
Both [RFC4509] and [I-D.ietf-dnsext-dnssec-rsasha256] should also be
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considered part of the DNS Security Document Family as described by
[RFC4033], Section 10.
3. Significant Concerns
This section provides clarifications that, if overlooked, could lead
to security issues or major interoperability problems.
3.1. Clarifications on Non-Existence Proofs
[RFC4035] Section 5.4 underspecifies the algorithm for checking non-
existence proofs. In particular, the algorithm as presented would
incorrectly allow an NSEC or NSEC3 RR from an ancestor zone to prove
the non-existence of other RRs at that name in the child zone or
other names in the child zone.
An "ancestor delegation" NSEC RR (or NSEC3 RR) is one with:
o the NS bit set,
o the SOA bit clear, and
o a signer field that is shorter than the owner name of the NSEC RR,
or the original owner name for the NSEC3 RR.
Ancestor delegation NSEC or NSEC3 RRs MUST NOT be used to assume non-
existence of any RRs below that zone cut, which include all RRs at
that (original) owner name other than DS RRs, and all RRs below that
owner name regardless of type.
Similarly, the algorithm would also allow an NSEC RR at the same
owner name as a DNAME RR, or an NSEC3 RR at the same original owner
name as a DNAME, to prove the non-existence of names beneath that
DNAME. An NSEC or NSEC3 RR with the DNAME bit set MUST NOT be used
to assume the non-existence of any subdomain of that NSEC/NSEC3 RR's
(original) owner name.
3.2. Validating Responses to an ANY Query
[RFC4035] does not address how to validate responses when QTYPE=*.
As described in Section 6.2.2 of [RFC1034], a proper response to
QTYPE=* may include a subset of the RRsets at a given name -- it is
not necessary to include all RRsets at the QNAME in the response.
When validating a response to QTYPE=*, validate all received RRsets
that match QNAME and QCLASS. If any of those RRsets fail validation,
treat the answer as Bogus. If there are no RRsets matching QNAME and
QCLASS, validate that fact using the rules in [RFC4035] Section 5.4
(as clarified in this document). To be clear, a validator must not
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expect to receive all records at the QNAME in response to QTYPE=*.
3.3. Check for CNAME
Section 5 of [RFC4035] says little about validating responses based
on (or that should be based on) CNAMEs. When validating a NOERROR/
NODATA response, validators MUST check the CNAME bit in the matching
NSEC or NSEC3 RR's type bitmap. If the CNAME bit is set, the
validator MUST validate the CNAME RR and follow it, as appropriate.
3.4. Insecure Delegation Proofs
[RFC4035] Section 5.2 specifies that a validator, when proving a
delegation is not secure, needs to check for the absence of the DS
and SOA bits in the NSEC (or NSEC3) type bitmap. The validator also
needs to check for the presence of the NS bit in the NSEC (or NSEC3)
RR (proving that there is, indeed, a delegation). If this is not
checked, spoofed unsigned delegations might be used to claim that an
existing signed record is not signed.
3.5. Errors in Canonical Form Type Code List
When canonicalizing DNS names, DNS names in the RDATA section of NSEC
and RRSIG resource records are not downcased.
[RFC4034] Section 6.2 item 3 has a list of resource record types for
which DNS names in the RDATA are downcased for purposes of DNSSEC
canonical form (for both ordering and signing). That list
erroneously contains NSEC and RRSIG. According to [RFC3755], DNS
names in the RDATA of NSEC and RRSIG should not be downcased.
The same section also erroneously lists HINFO, and twice at that.
Since HINFO records contain no domain names, they are not subject to
downcasing.
4. Interoperability Concerns
4.1. Unknown DS Message Digest Algorithms
Section 5.2 of [RFC4035] includes rules for how to handle delegations
to zones that are signed with entirely unsupported algorithms, as
indicated by the algorithms shown in those zone's DS RRsets. It does
not explicitly address how to handle DS records that use unsupported
message digest algorithms. In brief, DS records using unknown or
unsupported message digest algorithms MUST be treated the same way as
DS records referring to DNSKEY RRs of unknown or unsupported
algorithms.
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The existing text says:
If the validator does not support any of the algorithms listed in
an authenticated DS RRset, then the resolver has no supported
authentication path leading from the parent to the child. The
resolver should treat this case as it would the case of an
authenticated NSEC RRset proving that no DS RRset exists, as
described above.
To paraphrase the above, when determining the security status of a
zone, a validator discards (for this purpose only) any DS records
listing unknown or unsupported algorithms. If none are left, the
zone is treated as if it were unsigned.
Modified to consider DS message digest algorithms, a validator also
discards any DS records using unknown or unsupported message digest
algorithms.
4.2. Private Algorithms
As discussed above, section 5.2 of [RFC4035] requires that validators
make decisions about the security status of zones based on the public
key algorithms shown in the DS records for those zones. In the case
of private algorithms, as described in [RFC4034] Appendix A.1.1, the
eight-bit algorithm field in the DS RR is not conclusive about what
algorithm(s) is actually in use.
If no private algorithms appear in the DS set or if any supported
algorithm appears in the DS set, no special processing will be
needed. In the remaining cases, the security status of the zone
depends on whether or not the resolver supports any of the private
algorithms in use (provided that these DS records use supported hash
functions, as discussed in Section 4.1). In these cases, the
resolver MUST retrieve the corresponding DNSKEY for each private
algorithm DS record and examine the public key field to determine the
algorithm in use. The security-aware resolver MUST ensure that the
hash of the DNSKEY RR's owner name and RDATA matches the digest in
the DS RR. If they do not match, and no other DS establishes that
the zone is secure, the referral should be considered BAD data, as
discussed in [RFC4035].
This clarification facilitates the broader use of private algorithms,
as suggested by [RFC4955].
4.3. Caution About Local Policy and Multiple RRSIGs
When multiple RRSIGs cover a given RRset, [RFC4035] Section 5.3.3
suggests that "the local resolver security policy determines whether
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the resolver also has to test these RRSIG RRs and how to resolve
conflicts if these RRSIG RRs lead to differing results." In most
cases, a resolver would be well advised to accept any valid RRSIG as
sufficient. If the first RRSIG tested fails validation, a resolver
would be well advised to try others, giving a successful validation
result if any can be validated and giving a failure only if all
RRSIGs fail validation.
If a resolver adopts a more restrictive policy, there's a danger that
properly-signed data might unnecessarily fail validation, perhaps
because of cache timing issues. Furthermore, certain zone management
techniques, like the Double Signature Zone-signing Key Rollover
method described in section 4.2.1.2 of [RFC4641] might not work
reliably.
4.4. Key Tag Calculation
[RFC4034] Appendix B.1 incorrectly defines the Key Tag field
calculation for algorithm 1. It correctly says that the Key Tag is
the most significant 16 of the least significant 24 bits of the
public key modulus. However, [RFC4034] then goes on to incorrectly
say that this is 4th to last and 3rd to last octets of the public key
modulus. It is, in fact, the 3rd to last and 2nd to last octets.
4.5. Setting the DO Bit on Replies
[RFC4035] does not provide any instructions to servers as to how to
set the DO bit. Some authoritative server implementations have
chosen to copy the DO bit settings from the incoming query to the
outgoing response. Others have chosen to never set the DO bit in
responses. Either behavior is permitted. To be clear, in replies to
queries with the DO-bit set servers may or may not set the DO bit.
4.6. Setting the AD bit on Replies
Section 3.2.3 of [RFC4035] describes under which conditions a
validating resolver should set or clear the AD bit in a response. In
order to protect legacy stub resolvers and middleboxes, validating
resolvers SHOULD only set the AD bit when a response both meets the
conditions listed in RFC 4035, section 3.2.3, and the request
contained either a set DO bit or a set AD bit.
Note that the use of the AD bit in the query was previously
undefined. This document defines it as a signal indicating that the
requester understands and is interested in the value of the AD bit in
the response. This allows a requestor to indicate that it
understands the AD bit without also requesting DNSSEC data via the DO
bit.
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4.7. Setting the CD bit on Requests
When processing a request with the CD bit set, the resolver MUST set
the CD bit on its upstream queries.
4.8. Nested Trust Anchors
A DNSSEC validator may be configured such that, for a given response,
more than one trust anchor could be used to validate the chain of
trust to the response zone. For example, imagine a validor
configured with trust anchors for "example." and "zone.example."
When the validator is asked to validate a response to
"www.sub.zone.example.", either trust anchor could apply.
When presented with this situation, DNSSEC validators SHOULD try all
applicable trust anchors until one succeeds.
There are some scenarios where different behaviors, such as choosing
the trust anchor closest to the QNAME of the response, may be
desired. A DNSSEC validator MAY enable such behaviors as
configurable overrides.
5. Minor Corrections and Clarifications
5.1. Finding Zone Cuts
Appendix C.8 of [RFC4035] discusses sending DS queries to the servers
for a parent zone. To do that, a resolver may first need to apply
special rules to discover what those servers are.
As explained in Section 3.1.4.1 of [RFC4035], security-aware name
servers need to apply special processing rules to handle the DS RR,
and in some situations the resolver may also need to apply special
rules to locate the name servers for the parent zone if the resolver
does not already have the parent's NS RRset. Section 4.2 of
[RFC4035] specifies a mechanism for doing that.
5.2. Clarifications on DNSKEY Usage
Questions of the form "can I use a different DNSKEY for signing this
RRset" have occasionally arisen.
The short answer is "yes, absolutely". You can even use a different
DNSKEY for each RRset in a zone, subject only to practical limits on
the size of the DNSKEY RRset. However, be aware that there is no way
to tell resolvers what a particularly DNSKEY is supposed to be used
for -- any DNSKEY in the zone's signed DNSKEY RRset may be used to
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authenticate any RRset in the zone. For example, if a weaker or less
trusted DNSKEY is being used to authenticate NSEC RRsets or all
dynamically updated records, that same DNSKEY can also be used to
sign any other RRsets from the zone.
Furthermore, note that the SEP bit setting has no effect on how a
DNSKEY may be used -- the validation process is specifically
prohibited from using that bit by [RFC4034] section 2.1.2. It is
possible to use a DNSKEY without the SEP bit set as the sole secure
entry point to the zone, yet use a DNSKEY with the SEP bit set to
sign all RRsets in the zone (other than the DNSKEY RRset). It's also
possible to use a single DNSKEY, with or without the SEP bit set, to
sign the entire zone, including the DNSKEY RRset itself.
5.3. Errors in Examples
The text in [RFC4035] Section C.1 refers to the examples in B.1 as
"x.w.example.com" while B.1 uses "x.w.example". This is painfully
obvious in the second paragraph where it states that the RRSIG labels
field value of 3 indicates that the answer was not the result of
wildcard expansion. This is true for "x.w.example" but not for
"x.w.example.com", which of course has a label count of 4
(antithetically, a label count of 3 would imply the answer was the
result of a wildcard expansion).
The first paragraph of [RFC4035] Section C.6 also has a minor error:
the reference to "a.z.w.w.example" should instead be "a.z.w.example",
as in the previous line.
5.4. Errors in RFC 5155
A NSEC3 record, that matches an Empty Non-Terminal, effectively has
no type associated with it. This NSEC3 record has an empty type bit
map. Section 3.2.1 of [RFC5155] contains the statement:
Blocks with no types present MUST NOT be included.
However, the same section contains a regular expression:
Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )+
The plus sign in the regular expression indicates that there is one
or more of the preceding element. This means that there must be at
least one window block. If this window block has no types, it
contradicts with the first statement. Therefore, the correct text in
RFC 5155 3.2.1 should be:
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Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )*
6. IANA Considerations
This document specifies no IANA Actions.
7. Security Considerations
This document does not make fundamental changes to the DNSSEC
protocol, as it was generally understood when DNSSECbis was
published. It does, however, address some ambiguities and omissions
in those documents that, if not recognized and addressed in
implementations, could lead to security failures. In particular, the
validation algorithm clarifications in Section 3 are critical for
preserving the security properties DNSSEC offers. Furthermore,
failure to address some of the interoperability concerns in Section 4
could limit the ability to later change or expand DNSSEC, including
by adding new algorithms.
8. References
8.1. Normative References
[I-D.ietf-dnsext-dnssec-rsasha256]
Jansen, J., "Use of SHA-2 algorithms with RSA in DNSKEY
and RRSIG Resource Records for DNSSEC",
draft-ietf-dnsext-dnssec-rsasha256-10 (work in progress),
January 2009.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
RFC 1034, STD 13, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
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Extensions", RFC 4035, March 2005.
[RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
(DS) Resource Records (RRs)", RFC 4509, May 2006.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
8.2. Informative References
[RFC3755] Weiler, S., "Legacy Resolver Compatibility for Delegation
Signer (DS)", RFC 3755, May 2004.
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, September 2006.
[RFC4955] Blacka, D., "DNS Security (DNSSEC) Experiments", RFC 4955,
July 2007.
Appendix A. Acknowledgments
The editors would like the thank Rob Austein for his previous work as
an editor of this document.
The editors are extremely grateful to those who, in addition to
finding errors and omissions in the DNSSECbis document set, have
provided text suitable for inclusion in this document.
The lack of specificity about handling private algorithms, as
described in Section 4.2, and the lack of specificity in handling ANY
queries, as described in Section 3.2, were discovered by David
Blacka.
The error in algorithm 1 key tag calculation, as described in
Section 4.4, was found by Abhijit Hayatnagarkar. Donald Eastlake
contributed text for Section 4.4.
The bug relating to delegation NSEC RR's in Section 3.1 was found by
Roy Badami. Roy Arends found the related problem with DNAME.
The errors in the [RFC4035] examples were found by Roy Arends, who
also contributed text for Section 5.3 of this document.
The editors would like to thank Ed Lewis, Danny Mayer, Olafur
Gudmundsson, Suzanne Woolf, and Scott Rose for their substantive
comments on the text of this document.
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Authors' Addresses
Samuel Weiler
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, Maryland 21046
US
Email: weiler@tislabs.com
David Blacka
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166
US
Email: davidb@verisign.com
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